1. Field of the Invention
The present invention relates to a radio selective call receiver, and more particularly to a radio selective call receiver for receiving a radio signal of a signal format having a plurality of sync signals in one frame.
2. Description of the Related Art
With rapid spread of radio selective call services and liberalization of services, increase in a number of lines and in the data transfer rate has been recently intended, and frequencies of electric waves used for the radio selective call services tend to increase. Under such circumstances, when an electric field intensity of a desired wave received by its own radio selective call receiver is smaller than that of a wave received by any other radio selective call receiver, the latter wave serves as an interfering wave. There occurs such a problem as that the interfering wave extremely degrades a receiving ratio of the radio selective call receiver by intermodulation of a plurality of waves.
Two techniques have been used as a prior art for solving the above problem. A first technique lowers a level of a radio signal to prolong a preamble search mode when a bit synchronization can not be achieved due to the interfering wave. A second technique lowers a level of the radio signal received according to the receive synchronization enabled/disabled state.
The radio selective call receiver in the first technique is disclosed in, for example, U.S. patent application Ser. No. 08/365434 (Japanese laid-open patent application heisei 5-336605).
With this radio selective call receiver, a controller turns on an attenuation circuit to increase a number of bits of a preamble search mode by a predetermined number if the controller can not achieve the bit synchronization and determines that the interfering wave is generated from an output of a comparator. When the attenuation circuit is turned on, a level of the radio signal is attenuated, improving the receiving rate.
In this radio selective call receiver, however, when the interfering wave is generated, a number of bits of the preamble search mode must be increased and the efficiency of a receiver power supply can not be therefore enhanced.
On the other hand, a radio selective call receiver in the second technique is disclosed in, for example, U.S. Pat. No. 5,369,799.
In this radio selective call receiver, a decoder detects a frame-sync signal and a group-sync signal from a received signal, thereby achieving the receive synchronization. If at least either the frame-sync signal or the group-sync signal is not detected, the decoder outputs a control signal. A gain control circuit controls gain allocation of the received signal based on the control signal from the decoder.
In this radio selective call receiver, however, allocation of the gain of the radio signal input to a receiver section in a second frame is changed when the frame-sync signal and the group-sync signal are not detected in a first frame, and hence the receiving opportunity is lost for the first frame.